Ozone gas exhibits various absorption bands in the UV, visible and infrared spectra. The most prominent absorption band occurs in the UV range around 254 nm, the Hartley band. This band has long been used in absorption spectrophotometers for ozone measurement, and UV absorption has become a common, proven analytical method to determine ozone concentrations. UV absorption is also recognized by many international entities (such as IOA, EPA and NIST) as the Standard Method for measuring gas-phase ozone.
The use of UV absorption photometry for ozone measurement derives from a number of factors. The strong absorption at 254 nm lends to instruments with small-geometry and very wide dynamic range. In addition, reliable light sources that produce the desired discrete spectrum are readily available in the form of low pressure mercury vapor lamps.
Beer-Lambert Absorption Equation
The mathematical relationship between light absorption and gas density can be described by the following generalized equation: ##EQU1## where I is the integrated radiant power through the sample, (.lambda..sub.h -.lambda..sub.1) is the measurement spectral bandwidth, I.sub.0 (.lambda.) is the power spectrum measured at zero gas density, .epsilon. (.lambda.) is the absorption coefficient spectrum, L is the length of the optical path, and C is the ozone density in mass/volume. In practice, when the absorption coefficient is assumed to be constant within the measurement band or when the bandwidth is sufficiently narrow (approximating a single wavelength or monochromatic condition), the standard Beer-Lambert equations are used as follows: ##EQU2##
Typically, absorption photometers are designed to use equation (3) under the assumptions that .epsilon. and L are known constants and that I and I.sub.0 can be accurately measured. Once those assumptions are satisfied, the instrument calculates C as a linear function of the absorbance ln(I.sub.0 /I). Key issues which can lead to deviations from these basic assumptions and produce inaccurate measurements are discussed below.
Measurement of I.sub.0 and I
I.sub.0 represents the radiant power through the sample optical path with ozone free sample (zero density). The frequency at which I.sub.0 needs to be measured (frequency at which the instrument needs to be zeroed) depends on the stability of the optical system. I, the radiant power through the sample, represents the real-time signal in the presence of ozone in the cuvette.
In practical systems, the measurements will always include "noise" components that affect the accuracy of the estimated signals, and in turn the accuracy of the calculated ozone density. The "noise" level is typically constant and independent of gas density, while the signal level decreases exponentially with density. Therefore, at ultra-high densities, the signal-to-noise ratio drops exponentially, and correspondingly, the error in the calculated density increases rapidly. To alleviate this problem and attain high signal-to-noise ratio at the upper end of the instrument's range, it is essential to maintain high signal level at ultra-high densities.
Instability of the light source and detector, and changes in the optical characteristics of the cuvette due to for example soil buildup, solarization, etc., are other issues affecting the accuracy of the estimated signal. To compensate for these problems, it is essential to measure I.sub.0 very frequently.
Optical Path and Ultra-high Densities
Equations (1) through (3) assume a linear optical path of a predetermined length L. The linear requirement implies same optical path length for all light rays interacting with the absorbing species. The length of the optical path is a design parameter determined by tradeoffs among the specific absorption coefficients, the targeted ozone density range, the minimum signal-to-noise ratio (or precision and linearity at the upper end of the range), and the tolerance for zero drifts. Typically, the higher the density range, the higher the minimum signal-to-noise ratio, and the larger the absorption coefficient, a shorter optical path lengths and more stringent manufacturing tolerances are required. In addition, a short path length typically implies a tighter restriction for gas flow which presents a challenge if high flow rates are required. To address this issue and to optimize the above tradeoffs while delivering high precision performance, it is necessary to use spectra in which the targeted species exhibit lower absorption values.
High Density High Pressure Needs
Industrial applications of ozone now require very high ozone densities, in excess of .about.300 g/Nm.sup.3. Generator manufacturers are in fact meeting this demand with newly designed generators capable of delivering these ozone levels. In addition, many industrial applications require that ozone be delivered and measured at high pressures. All other factors being constant, the ozone density C, is a function of the pressure of the sample gas. Since compressed ozone typically behaves as an ideal gas, increasing the pressure by a factor of .alpha., would result in a measured density C.sub.P equal to (.alpha..times.C). For example, in bleaching paper pulp applications, the output of an ozone generator, typically .apprxeq.100 g/Nm.sup.3, is compressed to several atmospheres before it can successfully be used in a modified Kraft sequence. The ozone density of this compressed sample, C.sub.P, is therefore several hundred g/m.sup.3.
The Chappuis Band
As explained before, the length of the optical path is determined by tradeoffs among the absorption coefficient, the targeted ozone density range, the budgeted minimum signal-to-noise ratio, and the tolerance for zero drifts. When the targeted density is ultra high, these tradeoffs may result in impractical geometry for the cell or/and inadequate analyzer performance, for absorption in the Hartley band. In this case, an alternative approach is to operate the spectrophotometer at the ozone absorption peak in the Chappuis band around 604 nm.
The absorption coefficient at the Chappuis band peak is .epsilon.=0.055 cm.sup.-1 (for comparison, the absorption coefficient at 254 nm is 134.0 cm.sup.-1). This small value of .epsilon. leads to more practical gap/cell geometry. However, in order to measure ultra-high ozone density at a performance comparable to UV-based instruments requires extremely stable signals (I.sub.0 and I), high signal-to-noise figures, and very small zero drifts.